There are many instances in the search for and extraction of underground natural resources where one needs to have a representation of an underground geological boundary or even a topological surface. Even though the descriptions contained herein are to subsurface boundaries, the methodology is applicable to any geological or topological boundary. For example, the layout of the top of a hydrocarbon bearing rock formation may be of interest, the transition between zones of different porosity may be of interest, the transition between zones of different resistivity may be of interest, the transition of any physical or chemical property, or the transition between different formation types may of interest. Creating a mathematical representation of such a boundary is referred to as creating a “surface”, or sometimes referred to as creating a “horizon”.
Changes in depth or elevation of the geological boundary, such as caused by differential compaction, differential geological uplifting, and broken by faults, make the geological boundary and therefore the representative surface complex. In many cases, creating the surface is based on a limited data set, such as a limited number of actual depth or elevation values from actual boreholes or topological measurements, and known locations of geological faults. Geological faults represent what may be considered a step change in depth or elevation of the geological boundary, and thus faults have always presented difficulties in calculating a surface from a limited data set. In particular, in the related-art when interpolating depth values for the surface along a geological fault, the actual data values that reside across the fault are not used under the theory that data on the opposite side of the fault are not reliable predictions of depth because of the geological fault. Stated otherwise, related-art methods of calculating fault throws do not “look” across the fault for depth values to use in the interpolations.
The related-art rule of not looking across the fault creates difficulty in situations where a geological boundary resides between two geological faults. Because of the faults, and the rule of not looking across a fault for depth data, either no depth data may be available for a particular zone, or depth data may be far removed from the location of the zone such that the calculated depth in the zone between the two geological faults will be significantly higher or lower than could be expected for the actual geological boundary.
Any advance which can be used to more closely estimate the location of an underground geological boundary would provide a competitive advantage in the marketplace.